1. Field
This patent specification relates to a wobble signal detecting circuit in use primarily for optically recording and reading out, and more particularly, to such circuits capable of effectively detecting wobble signals prerecorded in an optical recording medium and being suitably incorporated into an optical information record and readout system.
2. Discussion of the Background
Optical information record/readout systems (or optical disc systems) have come into practical use for viable information data storage and archival device of large capacity, which are configured to record information into recording regions formed continuously in the shape of spiral on an optical recording medium and read out recorded information from the medium with laser beams emanated from an optical pickup device.
With recent improvements in overall capabilities, it has become feasible of a personal computer to process AV (audio-visual) information such as music sounds and movie images, for example. Since the data volume of AV information is considerably large, optical recording media has been attracting much attention as viable means for storing such information data, and optical disc systems with lowering costs have been in wide spread use as one of peripherals of personal computers.
As described earlier, for rewritable recording media such as CD-R (CD-Recordable) and CD-RW (CD-Rewritable), a plurality of recording regions are formed in general with tracks (or pregrooves) formed continuously on an optical recording medium. In addition, by meandering (or wobbling) the tracks, additional pieces of information pertinent to driving the recording media is recorded as wobbling signals.
Of particular importance among the pieces of information is ATIP (Absolute Time in Pregroove) information. This information includes time information designating absolute address on recording medium (absolute time information) for properly controlling the location of the optical pickup during recording and reading out process steps. In addition, the ATIP information also includes signals for synchronizing overall operation with the velocity of disc rotation.
If correct ATIP information is not obtained, therefore, the control of optical pickup to bring to a proper programmed location can not be achieved, nor the necessary synchronization of the overall operation with disc rotation, which may give rise to recording errors.
These errors have serious effects particularly on the CD-R disc currently used, in which the disc has to be prohibited from a further use if one recording error is once detected.
Therefore, it is of considerable importance to obtain correct ATIP information during recording and reading out process steps. This becomes feasible by detecting wobble signals with a high accuracy.
The wobble signals have been detected previously by receiving light beams reflected from recording tracks with a photoreceptive device doubly split (or doubly-split photoreceptor) in the tangential direction with respect to the recording track regions, obtaining tracking error signals based on photoelectric signals (output signals) from respective portions of the doubly-split photoreceptive device, and extracting predetermined frequency components from the tracking error signals.
The signals obtained from reflected light beams, however, have to be examined closely because of the effect from the beam intensity as described herein below.
That is, the power emanated from a semiconductor laser in optical pickup is increased during recording into an optical disc. During the periods in-between recordings (i.e., space time without recording), in contrast, the power is decreased to the approximately same level of magnitude as that for readout periods. As a result, the power of laser emission varies in a pulse-shaped manner with time. The photoelectric signals output from photoreceptive device, however, do not exhibit the same pulse shape.
For example, in the case of approximately quad-nominal (CD 4×) speed, the intensity of photoelectric signals output from photoreceptive device exhibits a peak almost immediately after the increase in laser power (i.e., after showing leading increase with time) during recording, then decreases gradually with the formation of recorded marks, that is followed by flat portions which appears partially during the period corresponding to decreased laser power.
The change in photoelectric signal intensity, therefore, has a waveform different from that of the laser emission for the following reason: Chemical and other similar changes take place at the disc portions irradiated with laser beams following the formation of recorded marks, thereby giving rise to reflectivity change of corresponding track portions.
Because of such a rather complex change exhibited by signal intensity of reflected light, it has been often difficult to detect wobble signals from tracking error signals with a high accuracy.
In order to obviate the above noted difficulty, several detecting circuits have been disclosed for accurately detecting wobble signals during information recording into optical discs. One example of such circuit is Japanese Laid-Open Patent Application No. 2001-93147.
According to this reference, a wobble signal detecting circuit includes an optical detecting means for optically detecting a pair of portions (left- and right-hand sides) which are formed by splitting a light beam spot and aligned in the tangential direction with respect to the recording tracks, sample-holding means for carrying out sample-and-hold steps with respect to signals output from respective left- and right-hand sides of the beam spot, low range frequency filtering means for filtering noise components which are caused by filtering and included in respective signals output from left- and right-hand sides portions, and subtracting means for computing the difference between respective signals output from the low range frequency filtering means, whereby wobble signals are obtained.
In addition, the above noted sample-holding steps are carried out with respect to the aforementioned flat portions, which appears partially during the period corresponding to decreased laser power, in the intensity versus time curve of photoelectric signals.
With the wobble signal detecting circuit according to that disclosure, it appears feasible to detect wobble signals accurately in the case of relatively low recording velocity, since the flat regions in the intensity versus time curve of photoelectric signals are relatively stable for respective output signals from left- and right-hand side portions.
In the case of relatively high recording velocity, in contrast, the intensity versus time curve of photoelectric signals becomes more complicated for respective output signals, and the flat regions become less stable. As a result, output photoelectric signals include more complex noise components which can not be removed completely. This gives rise to a drawback, in which errors may be included in detected wobble signals.
Since time interval for carrying out sample-holding steps becomes shorter with increasing recording velocity, the sample-holding means is required to function stably and accurately at high recording velocities. This tends to hamper miniaturization and reducing costs of the wobble signal detecting circuit, to thereby give rise to another drawback.
In addition, with further increase in recording capacity of optical recording media in future, concomitant increase in recording velocity can be much anticipated which necessitates improvements in high frequency capabilities of the wobble signal detecting circuit.